Modular lighting application

ABSTRACT

A modular system comprising a first component comprising an LED driver and is second component comprising a light engine is described. —the LED driver comprising: a switched mode power converter configured to output a supply current; a first control unit configured to control a switch of the switched mode power converter, thereby controlling the supply current; —the light engine comprising: an LED assembly configured to receive the supply current, the LED) assembly comprising a plurality of LEDs and one or more switches arranged in series or in parallel, with one or more LEDs of the plurality of LEDs, and a second control unit configured to control the one or more switches of the LED assembly, thereby controlling an LED current through the plurality of LEDs; wherein the first control unit is configured to control an amplitude of the supply current, the second control unit is configured to control a duty cycle of the LED current through the plurality of LEDs and wherein the first and second control unit are configured to synchronize to switching operation of the switch of the switched mode power converter with a switching operation of the one or more switches of the light engine.

The invention relates to the field of lighting applications, inparticular LED based lighting applications.

BACKGROUND OF THE INVENTION

The present invention relates to LED based lighting applications.Typically, such lighting application comprise a power source or powerconverter that is configured to supply a current to an LED assemblycomprising one or more LEDs. In known applications, LEDs producing lightof different colors are often combined, in order to realize a lightsource having an adjustable color output. In order to realize such anadjustable color output, the relative intensities of LED of differentcolor are adjusted, e.g. by operating the LEDs of different color atdifferent duty cycles.

In general, an intensity of an LED based lighting application may beadjusted by adjusting the amplitude of the current through the LED or byoperating the LED at an adjustable duty cycle. In the latter case, thecurrent through the LED may be reduced to a lower value, e.g. zeroduring a particular percentage of the time, thus reducing the averageintensity. Such process, also referred to as duty cycle dimming, isperformed at a comparatively high frequency that is not visible for thehuman eye or which cannot be perceived by cameras or the like either.

At present, LED assemblies and power sources for driving the LEDassemblies, also referred to as LED drivers, are strongly linked. Assuch, it may be cumbersome to combine an LED driver with different LEDassemblies. Typical problems that may e.g. arise are the incapability ofan LED driver to address multiple groups of LEDs in an LED assembly orto attain or influence certain visual effects.

SUMMARY OF THE INVENTION

It would be desirable to provide in LED based lighting applicationshaving an improved flexibility and modularity.

To better address these concerns, in a first aspect of the invention,there is provided a modular system comprising a first componentcomprising an LED driver and a second component comprising a lightengine;

-   -   the LED driver comprising:        -   a switched mode power converter configured to output a            supply current;        -   a first control unit configured to control a switch of the            switched mode power converter, thereby controlling the            supply current;    -   the light engine comprising:        -   an LED assembly configured to receive the supply current,            the LED assembly comprising a plurality of LEDs and one or            more switches arranged in series or in parallel with one or            more LEDs of the plurality of LEDs, and        -   a second control unit configured to control the one or more            switches of the LED assembly, thereby controlling an LED            current through the plurality of LEDs; wherein    -   the first control unit is configured to control an amplitude of        the supply current, the second control unit is configured to        control a duty cycle of the LED current through the plurality of        LEDs and wherein the first and second control unit are        configured to synchronize a switching operation of the switch of        the switched mode power converter with a switching operation of        the one or more switches of the light engine.

According to another aspect of the present invention, there is provideda modular system comprising a first component comprising an LED driverand a second component comprising a light engine;

-   -   the LED driver comprising:        -   a switched mode power converter configured to output a            supply current;        -   a first control unit configured to control a switch of the            switched mode power converter, thereby controlling the            supply current;    -   the light engine comprising:        -   an LED assembly configured to receive the supply current,            the LED assembly comprising a plurality of LEDs and one or            more switches arranged in series or in parallel with one or            more LEDs of the plurality of LEDs, and        -   a second control unit configured to control the one or more            switches of the LED assembly, thereby controlling an LED            current through the plurality of LEDs;    -   the system further comprising a main control unit configured to:    -   receive, at an input terminal, a set point representing a        desired illumination characteristic of the LED assembly:    -   determine, based on the received set point, a current amplitude        modulation scheme and a duty cycle modulation scheme;    -   provide a first output signal representative of the current        amplitude modulation scheme to the first control unit and a        second output signal representative of the duty cycle modulation        scheme to the second control unit;    -   wherein the first and second control unit are respectively        configured to apply the current amplitude modulation scheme and        the duty cycle modulation scheme within a modulation time        window, in order to generate the desired illumination        characteristic.

According to yet another aspect of the present invention, there isprovided a light engine comprising:

-   -   an LED assembly configured to receive a supply current from an        LED driver, the LED assembly comprising a plurality of LEDs and        one or more switches arranged in series or in parallel with one        or more LEDs of the plurality of LEDs, and    -   a control unit configured to control the one or more switches of        the LED assembly, thereby controlling an LED current through the        plurality of LEDs;    -   the control unit further being configured to:        -   receive, at an input terminal, a set point representing a            desired illumination characteristic of the LED assembly;        -   determine, based on the received set point, a current            amplitude modulation scheme and a duty cycle modulation            scheme;        -   output a first output signal representative of the current            amplitude modulation scheme for processing by an LED driver            control unit of the LED driver;    -   wherein the current amplitude modulation scheme represents an        amplitude of the supply current to be provided by the LED driver        as a function of time within a modulation time window and the        duty cycle modulation scheme represents switching operations for        the one or more switches as a function of time within the        modulation time window and wherein the current amplitude        modulation scheme and the duty cycle modulation scheme are        configured to, when applied by the LED driver control unit and        the control unit, to generate the desired illumination        characteristic.

According to yet another aspect of the present invention, there isprovided an LED driver comprising:

-   -   a switched mode power converter configured to output a supply        current for powering an LED assembly:    -   a control unit configured to control a switch of the switched        mode power converter, thereby controlling the supply current;    -   wherein the control unit is further configured to:        -   receive, at an input terminal of the control unit, LED            assembly information describing the LED assembly to be            powered;        -   receive, at the input terminal, a set point representing a            desired illumination characteristic to be generated, during            use, by the LED assembly;        -   determine, based on the received set point, a current            amplitude modulation scheme and a duty cycle modulation            scheme;        -   output a first output signal representative of the duty            cycle modulation scheme for processing by an LED assembly            control unit of the LED assembly that is to be powered;    -   wherein the current amplitude modulation scheme represents an        amplitude of the supply current to be provided by the LED driver        as a function of time within a modulation time window and the        duty cycle modulation scheme represents switching operations for        the LED assembly as a function of time within the modulation        time window and wherein the current amplitude modulation scheme        and the duty cycle modulation scheme are configured to, when        applied by the LED assembly control unit and the control unit,        to generate the desired illumination characteristic.

According to yet another aspect of the present invention, there isprovided a modular system comprising a first component comprising an LEDdriver and a second component comprising a light engine;

-   -   the LED driver comprising:        -   a switched mode power converter configured to output a            supply current;        -   a first control unit configured to control a switch of the            switched mode power converter, thereby controlling the            supply current;    -   the light engine comprising:        -   an LED assembly configured to receive the supply current,            the LED assembly comprising a plurality of LEDs or LED            groups and one or more switches arranged in series or in            parallel with one or more LEDs or LED groups of the            plurality of LEDs of LED groups, and        -   a second control unit configured to control the one or more            switches of the LED assembly, thereby controlling a supply            of the supply current to the plurality of LEDs or LED groups            of the LED assembly; wherein    -   the first control unit and the second control unit are        configured to co-operate and control the supply current and the        supply of the supply current to the LED assembly in accordance        with a desired illumination characteristic, and wherein the        first and second control unit are configured to synchronize a        switching operation of the switch of the switched mode power        converter with a switching operation of the one or more switches        of the light engine.

These and other aspects of the invention will be more readilyappreciated as the same becomes better understood by reference to thefollowing detailed description and considered in connection with theaccompanying drawings in which like reference symbols designate likeparts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a combination of an LED driver and light engine accordingto a first embodiment of the present invention.

FIG. 2 depicts a current amplitude modulation scheme and duty cyclemodulation schemes as can be applied in an embodiment of the presentinvention.

FIG. 3 depicts a modular system according to a second embodiment of thepresent invention.

FIG. 4 schematically depicts a modular system according to a thirdembodiment of the present invention.

FIG. 5 depicts a time division scheme as can be applied in an embodimentof the present invention.

FIG. 6 depicts a duty-cycle modulation scheme and corresponding timedivision scheme as can be applied in an embodiment of the presentinvention.

FIG. 7 depicts a Manchester coding of 4 bit symbols.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 depicts a modular system according to an embodiment of thepresent invention. The modular system as schematically shown comprises afirst component 200, i.e. an LED driver and a second component 210, i.e.a light engine.

Within the meaning of the present invention, light engine refers to acombination of an LED assembly and associated switches for controlling acurrent through the LEDs of the LED assembly. In general, the LEDassembly may comprise a plurality of LEDs arranged in a variety oftopologies. Further, the topology of the LEDs of the LED assembly mayalso be adjustable by means of switches, thereby e.g. changing atopology of two LEDs connected in series to a topology whereby the LEDsare connected in parallel.In accordance with the present invention, a light engine may furthercomprise a control unit or controller for controlling an operating stateof the switches of the light engine.Within the meaning of the present invention, an LED driver comprises aswitched mode power converter (SMPC) and a control unit for controllingthe switched mode power converter, in particular a switch, but notlimited to, of the switched mode power converter.In the embodiment as shown, the LED driver 200 comprises a switched modepower converter (SMPC) 220 and a first control unit 230 for controllingthe switched mode power converter (SMPC).In accordance with the present invention, various switched mode powerconverters may be applied such as Buck, Boost, Buck-Boost or hystereticconverters. In the embodiment as shown, the SMPC is a Buck converter 220including a diode 220.1 a power switch 220.2 and an energy storageelement 220.3, i.e. an inductance. Typically, such converters comprise aswitch such as switch 220.2 as shown, for controlling an output currentIs as supplied by the SMPC. In an embodiment, the SMPC 220 may e.g. bepowered via a rectified DC supply voltage 300.In the embodiment as shown, the light engine 210 is a separate componentcomprising an LED assembly 240, a plurality of switches 250 forcontrolling whether or not a current flows through the LEDs and acontrol unit 260 for controlling the switches. In the embodiment asshown, the LEDs of the LED assembly 240 are arranged in two parallelbranches 272, 274. The first branch 272 comprises three groups (group 1comprising LEDs A and B, group 2 comprising LED C and group 3 comprisingLED D) that are arranged in series, each group further having anassociated switch 280.1, 280.2, 280.3 for controlling the currentthrough the LED group. The LED assembly further comprises a secondbranch 274 comprising 4 LEDs, whereby LEDs 274.1 and 274.2 can beshorted by switch 280.4. Switches 280.5 and 280.6 control whether or nota current can be supplied to the respective branches 272 and 274.Note that in practice, for the topology as schematically shown in FIG.1, either one of the switches 280.5 or 280.6 should be closed. In caseboth switches would be open, the current path would not be closed. Itmay further be noted that, in case a plurality of parallel branches isapplied in the LED assembly, it may be preferred to ensure that, at eachinstant in time, only one serial switch such as switches 280.5 or 280.6is closed. By doing so, one can be sure that the entire current assupplied by the LED driver 200 is provided to the selected branch. Incase more than one serial switch would be closed at the same time, thesupply current Is would have to be distributed over the selectedbranches, i.e. the branches who's switches are closed.

Note however that, in case multiple parallel branches are connected tothe supply current, measures may be taken to ensure proper control ofthe current in each of the branches, thus enabling the applied currentsto result in a desired illumination set point. As an example of suchmeasures, a current equalisation may e.g. be done using current mirrorcircuits in which a current dictating branch is set to a current Isdivided by the number or connected parallel branches and wherebymultiple mirror branches are used to source or sink the current througheach of the connected parallel branches.

In accordance with an aspect of the present invention, the controllingof the SMPC, i.e. the power converter 220 of the LED driver 200 and ofthe switches 280.1-280.6 as applied in the light engine 210 is performedby separate control units.

Within the meaning of the present invention, a control unit orcontroller may e.g. be embodied as a microprocessor or processor or anyother type of control circuitry. In general, such a control unit maycomprise an input terminal 230.1, 260.1 for receiving command signalssuch as a user defined illumination set point, i.e. an input signal(e.g. provided via a user interface) representing a desired illuminationcharacteristic of the LED assembly. In an embodiment, such a desiredillumination characteristic may e.g. include a desired intensity and adesired color of the light as generated by the LED assembly of the lightengine or a certain ratio between the intensities or colors of multiplechannels or branches, or a dynamic sequence of such ratios, e.g.resulting in a light show. A control unit or controller may furthercomprise a processing unit for processing the commands or input signalsand e.g. a memory unit for storage of data. A control unit or controllerfurther typically has one or more output terminals 230.2, 260.2 foroutputting control signals, e.g. for controlling an electronic switch ofthe SMPC (indicated by the dotted line 230.3) or controlling a switch ofthe light engine (indicated by the dotted line 260.3).In a system as schematically shown in FIG. 1, a desired illuminationcharacteristic to be emitted by the LEDs of the LED assembly 240 can berealized as follows:In order to realize a desired illumination characteristic, e.g. aparticular color at a particular intensity, the current as provided bythe SMPC 220 can be modulated, i.e. the amplitude can be adjusted andthe duty cycle of the current through the different LEDs or LED groupsof the LED assembly can be adjusted, by switching of the switches 280.1to 280.6.In such embodiment, the control unit 230 thus controls the SMPC 220,thereby controlling the amplitude of the supply current Is as providedby the LED driver to the light engine, whereas the control unit 260 maybe configured to control the switches 280.1 to 280.6.In such a modular system as schematically shown in FIG. 1, a variety ofcontrol strategies may be implemented.As a first example, in an embodiment of the present invention, thesecond control unit 260 may act as the master control unit. In suchembodiment, the second control unit 260 may e.g. receive at an inputterminal, e.g. terminal 260.1, a set point representing a desiredillumination characteristic.As a second example, in an embodiment of the present invention, thefirst control unit 230 may act as the master control unit. In suchembodiment, the first control unit 230 may e.g. receive at an inputterminal, e.g. terminal 230.1, a set point representing a desiredillumination characteristic.As a third example, in an embodiment of the present invention, thesystem may comprise a separate master control unit (not shown) that isconfigured to receive, a set point representing a desired illuminationcharacteristic and, upon receipt of such a set point, process the setpoint and output commands to both the first and second control units,e.g. to the respective input terminals 230.1 and 260.1 of the first andsecond control units 230, 260. Regarding the third example, it mayfurther be mentioned that, in an embodiment, either the first or secondcontrol unit could be implemented as part of the master control unit orthe master control unit could be implemented as a part or component ofeither the LED driver or the light engine.As a fourth example, the first and second control units may have beendesigned to inter-operate such that they jointly execute an algorithmdefining the desired operations of the LED driver, in particular of theSMPC of the LED driver, and the LED assembly, in particular of theswitches of the LED assembly, each control unit thus having a differentbut mutually complimentary role.In an embodiment of the present invention, the desired illuminationcharacteristic is realized by appropriate switching and control of thecurrent during a time interval or time window referred to as themodulation time window. In an embodiment, the modulation time window isan interval, a particular period during which all required switchingactions and amplitude modulations of the current of the SMPC can begrouped, in order to realize the desired illumination characteristic.Phrased differently, the average intensity of the LEDs of the LEDassembly during the modulation time window is such that it correspondsto the desired intensity; the same holds for the desired color asindicated by the desired Illumination characteristic. By selecting themodulation time window sufficiently small, an observer will not noticethe actual modulation of the current or the switching of different LEDsor LED groups during the modulation time window. In such embodiment, theswitching operations as performed during the modulation time window areconsecutively repeated unit a new desired illumination set point orcharacteristic is desired. Note that, in case of a light show or aparticular lighting effect that includes a gradual, substantiallycontinuous change in an intensity and/or a color of the generatedillumination, a required intensity or color change may be accommodatedwithin the modulation time window as well.In an embodiment, the modulation time window may have a duration of 3.3msec or a multiple thereof. In an embodiment, the modulation time windowis sub-divided in multiple sub-windows whereby during each sub-window, aparticular purpose or task is aimed for. In such embodiment, one maye.g. arrange that each LED or LED group of the LED assembly of the lightengine is only operated in one sub-window. In an embodiment of thepresent invention, the particular purposes or tasks as performed duringthe sub-windows are such that, the combination of the tasks performedduring the modulation time window results in the realisation of anoverall purpose being achieved, the overall purpose e.g. being therealisation of a desired illumination characteristic, e.g. a desiredcolor and intensity characteristic as represented by a set point.The sub-windows as applied may have a variable length or may have afixed length.In an embodiment, a modulation time window of 3.3 msec is subdivided in8 sub-windows of 416 μsec.

In an embodiment, the implementation of the required control of the SMPCand of the switches of the LED assembly can be as follows:

In a first step, a set point or command representing a desiredillumination characteristic is received by the control unit acting asmaster control unit, i.e. either the first or second control unit or adedicated master control unit.Upon receipt of the set point or command, the control unit acting asmaster control unit may be configured to determine, based on the desiredillumination characteristic, a current amplitude modulation scheme and aduty cycle modulation scheme.In accordance with an embodiment of the present invention, the currentamplitude modulation scheme represents the amplitude of the current assupplied by the SMPC as a function of time and the duty cycle modulationscheme represents the required switching operations for the switches ofthe LED assembly. Note that, in an embodiment, the current amplitudemodulation scheme may consist of a sequence of non-zero current valuesto be supplied by the SMPC, see e.g. FIG. 2 below. However, in anembodiment, the switched mode power converter as applied in an LEDdriver as applied in the present invention, may be operated in a pulsedmode as well, whereby the current as supplied may be zero for apercentage of the time. Such an operation of the SMPC may e.g. bereferred to as pulse modulation and may be advantageously applied incase a comparatively low intensity of light is desired. In such case,e.g. where an intensity of less than 50% of the nominal intensity isrequired, it may be advantageous the ‘shut down’ the SMPC for half thetime rather than maintaining the current towards the light engine andapplying a duty cycle of less than 50% to the LEDs of the LED assembly.FIG. 2 schematically shows a possible current amplitude modulationscheme and corresponding duty cycle modulation schemes for the threegroups of the branch 272 of FIG. 1, the schemes indicating amplitudemodulations of the current Is as supplied and the required switchingoperations of the three groups of LEDs of the branch 272 during amodulation time window MTW. FIG. 2 schematically shows the switchingoperations of the parallel switches over the groups as indicated above.It is further assumed that the switch 280.5 is kept closed.As can be seen in the upper graph of FIG. 2, (indicating the supplycurrent Is as provided by the SMPC), the required current is at a valueI1 during intervals 0 to t1 and t2 to t3, whereas the required currentis at a value I2 during the interval t1 to t2.The three other graphs indicated by reference to the groups 1, 2, and 3,indicate whether or not the respective switches of the groups (switches280.1, 280.2 and 280.3) should be open or closed, whereby a non-zero maye.g. indicate that the switch should be open, a zero value of the graphindicating that the switch should be closed. During such periods, thecurrent as supplied, either I1 or I2 will flow through one or more ofthe parallel switches 280.1, 280.2 and 280.3. As such, in accordancewith the graph for group 2, one can see that switch 280.2 is only openduring the interval t1 to t4, during which interval the SMPC provides ina current with amplitude I2.By determining, based on a desired illumination characteristic that isto be generated by the LED assembly, a current modulation scheme andduty cycle modulation scheme as e.g. shown in FIG. 2, particularrequirements or constraints of the different LEDs as applied or thedifferent topologies as applied, may be taken into account more easily.In the example given, it may e.g. be that the LED C of group 2 may onlybe supplied with a current of amplitude I2, rather than a current I1. Asanother example, one could e.g. consider that LEDs A and B would beconnected in parallel rather than in series; in such a situation, thecurrent during the interval 0 to t1 could even be allowed to be higherthan I1 (assuming I1 being the nominal current of LEDs A, B and D),because, in the example shown, LEDs C and D are not on during thisinterval.

By distribution the processing or control power over the light engineand the LED driver, as e.g. done in an embodiment of the presentinvention, control of an LED assembly of a light engine is facilitated.In particular, by providing a control unit (such as control unit 260)‘on-board’ of a light engine, the processing of a command representing adesired Illumination characteristic can be performed at least partly bythe on-board control unit, said control unit having knowledge of the LEDassembly it is connected to. In such an arrangement, the control unit ofthe light engine may determine the desired current modulation scheme andswitching operations of available switches on the light engine, therebytaking into account any particulars, such as physical constraints ortopologies of the LED assembly. By incorporation this information in thecontrol unit of the light engine (e.g. in a memory unit of such acontrol unit), this information need not be shared or known to the LEDdriver that is used to power the light engine. In such an arrangement,the LED driver need not have any particular knowledge about anyconstraints imposed by the LED assembly as it is merely required, in anembodiment, to follow instructions as received by the control unit ofthe light engine. In particular, in such an embodiment, when the currentamplitude modulation scheme is determined, e.g. by the control unit ofthe light engine or a separate master control unit, this control unitmay provide the current amplitude modulation scheme to the control unitof the LED driver, e.g. in the form of a desired current set point, as afunction of time. As such, the control unit of the LED driver may thusact as a slave in a master-slave configuration with the control unit ofthe light engine and modulate the current as outputted as indicated bythe upper graph of FIG. 2, i.e. during the modulation time window.

As will be understood by the skilled person, the switching pattern andcurrent modulation pattern as indicated by the current amplitudemodulation scheme and the duty cycle schemes is subsequently repeateduntil a control unit of the system receives another illumination setpoint, which may then give rise to different current modulation andswitching schemes.

In an embodiment of the present invention, amplitude modulations of thecurrent amplitude modulation scheme and switching operations of the dutycycle modulation scheme are non-overlapping.

In this respect, it may be pointed out that, as will be understood bythe skilled person, LED driver as applied in the present invention maye.g. be equipped to generate more than one output current Is. Inparticular, LED drivers as applied in the present invention may e.g. beequipped with multiple power converter, each configured to generate oroutput, at an output channel or terminal of the LED driver, acontrollable current. In such embodiment, each SMPC may be controlled bya dedicated control unit, or a common control unit may be provided,controlling the operation of the multiple SMPCs. Such an embodiment ofan LED driver may e.g. be advantageously combined with a light enginehaving multiple channels. In an embodiment, such multiple channels maybe construed as multiple parallel branches, as e.g. done in FIG. 1.However, as an alternative, each branch of multiple parallel branchesmay have its own input terminal to receive a supply current such as thesupply current Is as shown in FIG. 1. As an example, an LED driver maythus have two output terminals outputting currents with a differentvalue, the output terminals being connected or connectable to two inputterminals connected or connectable to two different branches of LEDs. Itmay further be pointed out that in such an arrangement, whereby multipleoutput or supply currents are available and connectable to multipleinput terminals, one or more switches may be provided to selectivelyconnect the available branches of LEDs to the input terminals. As such,the connection of the LED branches, when available, to the multipleinput terminals may be varied in time as well. As such, an LED or LEDbranch may be connected to a first channel of the LED driver at aparticular instant or period in time and to a second, different channelat another instant or period in time. By doing so, intensity or colormodulations may be realized as well.In the embodiment as illustrated in FIG. 2, one can see that on t=t1,the current Is is modulated, i.e. changed from a value I1 to a value I2.At the same time, switch 280.2, associated with group 2, is switched toan open state. As will be understood by the skilled person, the currentas supplied or outputted by an SMPC will not change instantaneously froma value I1 to a different value I2. Rather, there will be a transient inthe current profile before the current is at the new set point value.During this transient, there is an uncertainty about the actual value ofthe current, rendering it difficult to determine the required switchingor duty cycling of the switches of the light engine, in order to avoidthis uncertainty, in an embodiment, the current modulations andswitching operations are separated in time. In FIG. 2, this is e.g.illustrated by the switching operation of group 3 (bottom graph of FIG.2). As can be seen, switch 280.3 associated with group 3 is switched toan open state (thus allowing the supply current Is to flow through theLED D) at t=t2+Δ, rather than at t=t2, when the current is changed froma value I2 to a value I1. Using such a delay Δ, one can ensure that thetransient behavior associated with a current modulation is over and thatthe current is at the actual expected value (e.g. I1), when the switchis operated. By means of the delay Δ. one can thus ensure that theswitching actions of the switches associated with the LEDs or LED groupsof the LED assembly are arranged to occur at instants in time that aredifferent from the instants at which the supply current of the SMPC isadjusted.In this respect, it can be pointed out that in case the LED driver, inparticular the SMPC of the LED driver also performs a pulse modulation,i.e. the output current Is is not continuous but pulsed, similar delaysmay be adequately applied with respect to the pulsed current asoutputted by the SMPC. With respect to the pulsed mode operation of theSMPC, it may be pointed out that and additional switch may be providedin the SMPC to turn off (in a pulsed mode) the SMPC.

In an embodiment of the present invention, the first and second controlunit are configured to synchronize a switching operation of a switch ofthe switched mode power converter (SMPC) with a switching operation ofone or more of the switches of the light engine. Due to the switchingoperation of the SMPC (e.g. by switching the switch 220.2 of the SMPC220 as shown in FIG. 1), the output current Is of an SMPC will not beconstant but will have a saw-tooth profile, as illustrated in the detail400 of the current Is. In the detail 400, instants ti indicate switchinginstants of the switch 220.2, said switching causing the current slopeto reverse. i.e. from an increasing current to a decreasing current andvice-versa).

By synchronizing the switching operations of the switches of the lightengine, as e.g. indicated by the duty cycle schemes shown in FIG. 2,with the switching instants ti of the SMPC, a more accuratecorrespondence between the actual illumination characteristic and thedesired characteristic can be achieved. In addition, parasiticdisturbances such as flicker may be mitigated by synchronizing theseinstants.In order to realize such a synchronization, various options exist.In an embodiment, the control units of the LED driver and of the lightengine as applied in the modular system according to the presentinvention, may e.g. in an embodiment, be provided with a common clocksignal to synchronize operations.FIG. 3 schematically shows a general set up of a modular system 500according to the present invention wherein various options forsynchronization are illustrated. The modular system 500 as showncomprises an LED driver 510 comprising a switched mode power converter(SMPC) 520 and a control unit 530 (e.g. a microprocessor ormicrocontroller or the like). The system 500 further comprises a lightengine 540 comprising an LED assembly 550 (comprising a plurality ofLED, e.g. arranged in groups and one or more switches for controllingthe currents through the LEDs or groups of LEDs of the LED assembly) anda control unit 560 for controlling the switches of the LED assembly 550.In embodiment as shown, lines 570 represent the power supply as providedby the SMPC to the light engine. Line 580 indicates a communicationchannel between the control unit 530 of the LED driver and the controlunit 560 of the light engine.

In the embodiment as shown, the system further comprises a mastercontrol unit 590 which may e.g. be configured to provide asynchronization signal 600 (e.g. a clock signal) to both the controlunits 530 and 560 to synchronize the switching. In such embodiment, thecontrol units of the LED driver and the light engine may thus make useof a common clock signal that is provided by the master control unit590. Alternatively, either the control unit of the LED driver or thecontrol unit of the light engine may provide a synchronization signal tothe other control unit, in order to synchronize operations. This cane.g. be realized via the communication channel 580. Communicationchannel 580 can be any suitable communication channel, either wired orwireless for exchanging data or commands between the control units 530and 560. The communication channel may be bi-directional oruni-directional, depending on the manner in which both control unitsco-operate. The communication channel may be of a hybrid form inconveying both analog waveforms to f.e. signal events or to conveyvalues or may exhibit digitally interpretable waveforms to conveycommands, status and data in digital form or any suitable combination ofthose. The communication channel can be synchronous, asynchronous,non-deterministic, deterministic, real-time or non-real time.

As an alternative to synchronizing via a common synchronization signalor clock signal, the light engine may e.g. be configured to detect theswitching instants ti of the switch of the SMPC. In FIG. 3, 620represents a measurement unit configured to measure the current assupplied by the SMPC and provide a signal 630 representative of thecurrent to the control unit 560 of the light engine. Based on such asignal, the control unit 560 may e.g. be configured to derive theswitching instants (instants ti as e.g. indicated in FIG. 2) of the SMPCand synchronize the switching of the switches of the light engine tothese instants.

Alternatively or in addition, as will be explained in more detail below,purposive pulses may be applied to enable a synchronisation of thecontrol units. As an example, the LED driver may be configured to applya current pulse at the start of every modulation time window.

In case the modulation time window is subdivided into multiplesub-windows, also referred to as slots or time slots, a current pulsemay also be applied at the start of each sub-window.

In an embodiment, different types of pulses may be applied to indicatethe start of the modulation time window or the start of a sub-window.Pulses may e.g. be differentiated based on their height, on theirposition or on their position relative to another pulse.

The current pulses as applied may also, in an embodiment, representparticular symbols to indicate the start of a modulation time window orsub-window. Such a representation of a symbol may be a predeterminedsequence of different current levels. As an example, of sequence ofhigh-low-high-low pulses may e.g. be interpreted as 1010 as a binarysymbol, whereas high-medium-low-high-low-medium may e.g. be interpretedas a multilevel symbol 210201.In an embodiment of the present invention, the application of the one ormore current pulses that are used for synchronisation purposes are suchthat they do not affect the average current or intensity as provided bythe LEDs or LED groups. Phrased differently, the contribution of acurrent pulse, either a positive or negative pulse can be accounted forin the current modulation scheme by compensating the positive ornegative pulse in the remainder of the modulation period, e.g. asub-window, that is used to generate a required current for a particularLED or LED group.

In an embodiment as described above, the control unit of the lightengine was configured to determine, based on a receive set pointrepresenting a desired illumination characteristic, a current amplitudemodulation scheme and a duty cycle modulation scheme.

In such an arrangement, the control unit of the light engine can beconsidered the master whereas the control unit of the LED driver acts asslave, following commands of the control unit of the light engine, e.g.received via a communication channel such as communication channel 580as shown in FIG. 3.In an alternative embodiment, the control unit of the LED driver, e.g.control unit 230 or control unit 530 acts as master control unit. Insuch embodiment, the control unit of the LED driver may e.g. beconfigured to:

-   -   receive, at an input terminal of the control unit, LED assembly        information describing the LED assembly to be powered and    -   receive, at the input terminal, a set point representing a        desired illumination characteristic to be generated, during use,        by the LED assembly.        As will be clear to the skilled person, in a modular system        whereby the LED driver and light engine are combined, the LED        driver may require information about the light engine that needs        to be powered. In accordance with the embodiment of the present        invention, the control unit of the LED driver may therefore be        configured to receive such information. In particular, such LED        assembly information may e.g. describe the topology or layout of        the LED assembly of the light engine that needs to be powered,        e.g. including technical data such as voltage or current        requirements of the different LED of the LED assembly,        descriptive data indicating how the LEDs are connected and        optionally grouped, descriptive data about the available        switches and the manner in which they control the current        through certain LEDs or LED groups of the LED assembly (e.g.        switches connected in series or in parallel).        In accordance with the embodiment of the present invention, the        control unit of the LED driver may then be configured to        determine, based on the received set point and the LED assembly        information, a current amplitude modulation scheme and a duty        cycle modulation scheme, in a similar manner as described above.        In such an arrangement, whereby the LED driver, in particular        the control unit of the LED driver, acts as master, may provide        a control signal to the control unit of the light engine, the        control signal representing the required duty cycle modulation        scheme to be executed by the light engine.        In such an embodiment, the LED driver may further communicate        certain events to the light engine, e.g. for the purpose of        synchronization. As an example, the LED driver may e.g.        communicate the occurrence of switching instants of the SMPC of        the LED driver to the light engine or the start of a modulation        time window as described in FIG. 2.

As an alternative to the use of the communication channel 580 as shownin FIG. 3, any required communication between the LED driver and thelight engine may also be realized by means of power line communicationor the like, whereby the power lines, e.g. power lines 570, are used forcommunicating data or commands between the LED driver and the lightengine. Such power line communication may e.g. include the use ofpositive or negative voltage or current pulses or spikes on the powerlines

By using a master-slave setup for the control units of the LED driverand the light engine, as e.g. discussed above, the communication betweenthe LED driver and the light engine may be kept to a minimum, since onlya current set points may need to be communicated, in an embodiment ofthe present invention.

In an embodiment, the modulation time window is considered to comprisesub-windows, one sub-window per group of LEDs that is controllable by aparticular switch, whereby the switching of the particular switch onlyoccurs in one of the sub-windows. In an embodiment, the sub-windows arenon-overlapping. In this respect, reference may e.g. be made to US2012/0235589, incorporated herein by reference in its entirety.

In an embodiment, an LED driver may e.g. be configured to supply acurrent to multiple LEDs or LED groups of a light engine during amodulation time window comprising a respective multiple sub-windows,whereby, during each sub-window, only one LED or LED group is suppliedwith the current.

A modular system according to the present invention, enabling suchoperation is schematically shown in FIG. 4. In the embodiment as shown,the LED driver 305 comprises a switched mode power converter (SMPC) 320and a first control unit 330 for controlling the switched mode powerconverter (SMPC).

In accordance with the present invention, various switched mode powerconverters may be applied such as Buck, Boost, Buck-Boost or hystereticconverters. In the embodiment as shown, the SMPC is a Buck converter 320including a diode 320.1 a power switch 320.2 and an energy storageelement 320.3, i.e. an inductance. Typically, such converters comprise aswitch such as switch 320.2 as shown, for controlling an output currentIs as supplied by the SMPC. In an embodiment, the SMPC 320 may e.g. bepowered via a rectified DC supply voltage 400. In an embodiment, theswitch may be controlled by the first control unit 330, e.g. based on acurrent measurement performed by the light engine, e.g. by detecting avoltage across a resistor in series with the LED assembly (not shown).In the embodiment as shown, the light engine 310 is a separate componentcomprising an LED assembly comprising multiple LEDs or LED groups, aplurality of switches for controlling whether or not a current flowsthrough the LEDs and a control unit 260 for controlling the switches. Inthe embodiment as shown, the LEDs of the LED assembly are arranged inone branch comprising three groups (group 1 comprising LEDs A and B,group 2 comprising LED C and group 3 comprising LED D) that are arrangedin series, each group further having an associated switch 380.1, 380.2,380.3 for controlling the current through the LED group. In accordancewith an aspect of the present invention, the controlling of the SMPC,i.e. the power converter 320 of the LED driver 305 and of the switches380.1-380.3 as applied in the light engine 310 may e.g. be performed byseparate control units.Within the meaning of the present invention, a control unit orcontroller may e.g. be embodied as a microprocessor or processor or anyother type of control circuitry. In general, such a control unit maycomprise an input terminal 330.1, 360.1 for receiving command signalssuch as a user defined illumination set point, i.e. an input signal(e.g. provided via a user interface) representing a desired illuminationcharacteristic of the LED assembly. In an embodiment, such a desiredillumination characteristic may e.g. include a desired intensity and adesired color of the light as generated by the LED assembly of the lightengine or a certain ratio between the intensities or colors of multiplechannels or branches, or a dynamic sequence of such ratios, e.g.resulting in a light show. A control unit or controller may furthercomprise a processing unit for processing the commands or input signalsand e.g. a memory unit for storage of data. A control unit or controllerfurther typically has one or more output terminals 330.2, 360.2 foroutputting control signals, e.g. for controlling an electronic switch ofthe SMPC (indicated by the dotted line 330.3) or controlling a switch ofthe light engine (indicated by the dotted line 360.3). In the embodimentas shown, a communication channel is further provided connecting thefirst and second control unit 330, 360. Such a communication channel maye.g. be a bidirectional or unidirectional serial communication channel.In a system as schematically shown in FIG. 4, a desired illuminationcharacteristic to be emitted by the LEDs of the LED assembly can berealized as follows during a modulation time window, under theassumption that only one of the LEDs or LED groups is on at the sametime.Assuming that a set point representing a desired intensity and color isreceived, e.g. via the input terminal 330.1 of the first control unit330. Based upon the desired color characteristic and the knowncharacteristics of the LED groups, the first control unit may thendetermine the appropriate required mixing of the LED group colors inorder to arrive at the desired color indicated by the set point. As anexample, the desired color may e.g. be realized by having the firstgroup (LEDs A and B) on for 25% of the time, having the second group(LED C) on for 50% of the time and having the third group (LED D) on for25% of the time. Note that these percentages are determined under theassumption of the same current being provided to the LED groups. Withinthe meaning of the present invention, these percentages may also bereferred to as ratios of the LED groups, indicative of the ON-time of anLED group over the modulation time window.

Once such an assessment is made, a modulation time window may, in anembodiment, be subdivided into different sub-windows having durations orperiods proportional to the determined percentages. Such a time-divisionof the modulation time window may be referred to as a time-divisionscheme, such a scheme representing the desired or required switchingactions required to apply the supply current to the appropriate LEDgroup, during the appropriate period.

In an embodiment of the present invention, the desired illuminationcharacteristic is realized by appropriate switching and control of thecurrent during a time interval or time window referred to as themodulation time window. In an embodiment, the modulation time window isan interval, a particular period during which all required switchingactions and amplitude modulations of the current of the SMPC can begrouped, in order to realize the desired illumination characteristic.Phrased differently, the average intensity of the LEDs of the LEDassembly during the modulation time window is such that it correspondsto the desired intensity; the same holds for the desired color asindicated by the desired illumination characteristic. By selecting themodulation time window sufficiently small, an observer will not noticethe actual modulation of the current or the switching of different LEDsor LED groups during the modulation time window.FIG. 5 schematically shows the required switching operations of theswitches 380.1, 380.2 and 380.3, LG-SS (LED group switching sequence),in order to realise the ratios as indicated for the LED groups, i.e. tosubdivide the modulation time window in accordance with a time-divisionscheme that results in a desired color. In particular, in order to turnon Group 1 of the LEDs during ¼^(th) of the period MTW (the modulationtime window), switch 380.1 needs to be open during the period T1 fromt=0 to t=t1; in order to turn on Group 2 of the LEDs during ½^(th) ofthe period MTW, switch 380.2 needs to be open during the period T2 fromt=t1 to t=t2, and, in order to turn on Group 3 of the LEDs during ¼^(th)of the period MTW, switch 380.3 needs to be open during the period T3from t=t2 to t=t3. In an embodiment, the control unit 360 of the lightengine may e.g. be configured to determine the desired time-divisionscheme needed to realise the desired color set point. In suchembodiment, the first control unit 330 may e.g. communicate the desiredcolor set point to the second control unit 360. Alternatively, the firstcontrol unit 330 may determine the desired time-division scheme andcommunicate it, e.g. via a serial communication channel. e.g. channel370, to the second control unit 360.

With respect to the desired intensity, e.g. indicated by a set point asreceived, such a desired intensity can be realised by supplying theappropriate current to the LED assembly of the light engine. In theexample as shown in FIG. 5, a current I=In, e.g. the nominal current ofthe LED driver, is provided to the light engine during the modulationtime period.

In this respect, it can be pointed out that, due to the application ofthe time-division scheme which represents the required sub division of amodulation time window to arrive at a desired color, the current can bemaintained at the same level during the entire modulation time window.One could represent such a situation by a current modulation scheme thatmerely represents a single value, In.

In another embodiment however, the current as supplied during eachsub-window may be changed as well. As an example, the applied supplycurrent may be different in each sub-window. In addition, currentmodulations, e.g. duty cycling may be applied within one or moresub-windows. As such, each sub-window may further be subdivided intodifferent sub-sub-windows, during which a particular current modulationmay be applied.

In case the intensity of the generated light is to be reduced, whilemaintaining the color set point, the current as generated by the LEDdriver, e.g. LED driver 305, can be reduced.

Alternatively, or in addition, the current as supplied by the LED drivercould be modulated.By performing the duty cycle modulation by the switch of the LED driver,the complexity of the time-division scheme can be kept low.Assuming that the intensity of the illumination needs to be reduced to50% and assuming a linear relationship between the intensity and thesupply current I, such a dimming set point can be realised by eitherreducing the current to 50% of the nominal current. In or by applying aduty cycle of 50% during each of the periods T1, T2, T3.In the latter case, in accordance with an embodiment of the presentinvention, the duty cycling may be realised by appropriate switching ofthe power switch 320.2 of the LED driver 305. In this respect, it can bepointed out that, when a very low intensity is required, e.g. lower thatan intensity corresponding to a lowest current value that can berealised by the LED driver, the application of duty cycling of the powerswitch of the LED driver may enable to realise this.FIG. 6 schematically shows a possible switching sequence, or duty cyclemodulation scheme, of the power switch 320.2, PS-SS, illustrating that asupply current is provided by the LED driver during a first half of theperiod T1, during a first half of the second period T2 and during afirst quarter of the third period T3. Assuming that the supply currentin the depicted situation corresponds to the lowest available current ofthe LED driver, the duty cycling of the power switch enables to obtaineven lower illumination set points.Note that, in the embodiment as shown, the switches of the light engine,i.e. switches 380.1-380.3 are merely applied to ensure that the currentsupplied by the LED driver is applied to the appropriate LED group,whereas the control of the current, both with respect to amplitude andduty cycle, is performed by the LED driver.Note that, in addition to performing a duty cycling using the powerswitch 320.2, and additional duty cycling using the switches 380.1-380.3could be considered as well, in a similar manner as illustrated in FIG.2.

It can further be noted that, in addition to the duty cycling of thepower switch 320.2 as e.g. shown in the switching scheme PS-SS of FIG.6, a current modulation of the supply current of the LED driver may beapplied as well; in particular, the current I as supplied need not bekept at the same value during the periods T1, T2, T3, the current maye.g. be kept at a first value I1 during a first half of the periods andat a second value I2 during the second half of the periods, thusenabling to obtained a higher resolution with respect to intensity.

In the embodiment as shown in FIGS. 5,6, the modulation time window issubdivided into sub-windows, whereby the duration of the sub-windows isselected to represent a desired color set point. Alternatively, thesub-windows may have a fixed period, e.g. 1/N×MTW, whereby N equals thenumber of LED groups. Note that, in such an embodiment, the current assupplied during the different sub-windows may need to be modulated, i.e.have different values, in order to arrive at a desired color. Thismodulation may e.g. include applying different current values during thedifferent sub-windows or applying a different duty cycle during thedifferent sub-windows, or applying a modulated current waveform duringthe different sub-windows or sub-sub-windows.

As such, in the embodiment as described in FIGS. 4-6, a desiredillumination set point may be obtained by determining a time divisionscheme, indicating a desired switching of the light engine, a duty cyclemodulation scheme and/or current modulation scheme, indicating a desiredoperation of the LED driver to generate the required supply current.

In a similar manner as described w.r.t. FIGS. 1-3, the processing orcontrol power to arrive at the set point may be distributed over thelight engine and the LED driver. In particular, in the example given,the control to arrive at the desired time-division scheme may berealised by the control unit 360 of the light engine, whereas thecontrol to arrive at the desired supply current, e.g. represented by thecurrent modulation scheme and/or duty cycle modulation scheme, e.g.modulation scheme PS-SS, may be realised by the control unit 320 of theLED driver. Alternative arrangements whereby the duty-cycle modulationis performed by the light engine or where the light engine may affectthe intensity emitted may be considered as well.

A consequence of the distribution of the control power over the controlunits of the LED driver and the light engine, is that an accuratesynchronisation between the control actions of the control units isdesired. Note that a synchronisation may be less critical in case one ofthe first or second control unit acts as a master controlling the othercontrol unit. In such arrangement, the master control unit may e.g.transmit a set point to the slave control unit.

The present invention proposes various ways to arrive at such asynchronisation.

The main objective of the synchronisation of the control actions of thecontrol units applied is to ensure that the illumination as generated bythe LED assembly of the light engine substantially matches a desiredillumination characteristic. e.g. represented by a set point, e.g.received by either the control unit of the LED driver (230, 330) or thecontrol unit of the light engine (260, 360). In this respect it can benoted that the control actions by the control unit may refer to controlsignals for controlling the switches in the LED driver, e.g. the switch220.2 or 320.2 of the SMPC of the LED driver, and/or to control signalsfor controlling the switches of the light engine, e.g. switches280.1-280.6 or 380.1-380.3. These control actions result in a modulationof the supply current as provided by the LED driver and a modulation ofthe current as supplied to the multiple LEDs or LED groups of the LEDassembly of the light engine.Since these actions may be initiated by different control units, asynchronisation is required.More specifically, the overall modulation as applied to the LEDs or LEDgroups is characterised by a sequence of modulation cycles, e.g.represented above as modulation time windows (MTW), where during eachmodulation time window, the currents through the LEDs or LED groups ismodulated in such manner that, on average, a desired intensity and/ordesired color is obtained. During such a modulation time window MTW, theinstantaneous current through a particular LED or LED group may varysubstantially, in accordance with the applied current modulation scheme,duty cycle modulation scheme or time-division scheme. In addition to theoverall, averaged, boundary conditions for the currents as supplied tothe LEDs or LED groups, there may be some additional requirements forthe instantaneous current, e.g. with respect to the occurrence offlicker or with respect to efficiency of the light engine, the LEDdriver or the modular system as a whole.

In an embodiment, the modulation time window as applied may further bedefined by it being subdivided in periods, referred to as sub-windows,where, during each period, a specific purpose or objective is targeted.A possible purpose is that during such a sub-window, as e.g. illustratedin FIGS. 5 and 6, a particular LED or LED group is controlled (the LEDor LED group e.g. radiating red light) while during the next sub-windowanother LED or LED group is controlled (that e.g. radiates blue light),and so on. A further purpose of the sub-windows may be to distribute thecontrol of a specific LED group in time such that the frequency contentof the applied current modulation for that specific LED group only holdsfrequencies that are sufficiently high, such that they are not observedin the overall generated illumination by a human or camera.

It may further be noted that the synchronization of the control units ofthe LED driver and the light engine should not only synchronize thestart and stop times of a modulation time window and its periods orsub-windows, which is further on referred to as ‘frequencysynchronisation’, there should also be a correct ‘phase relationship’between the applied control actions and schemes in order to ensure that,when a particular objective of the modular system is to be realisedduring a sub-window, e.g. LED group x is to be supplied with aparticular current and duty cycle, the control actions controlling thesupply current and the control actions controlling the sub-window are inphase. This synchronisation is further on referred to as ‘phasesynchronisation’. Without such a synchronisation, a first control unitmay e.g. target a purpose that is not in line with actions taken by thesecond control unit, despite being synchronised with respect tofrequency.

To illustrate this, reference can e.g. be made to the duty cyclemodulation scheme PS-SS and the time-division scheme FIG. 6. As can beseen, during T3 of the MTW, a current with a duty cycle of 25% issupplied to group 3. It can be pointed out however that the duty cycleof the current is implemented by the LED driver, whereas the enabling ofthe current through a particular LED or LED group is implemented by thelight engine. So, in order to apply the appropriate current to theappropriate LED or LED group, the duty cycle modulation scheme needs tobe in phase with the time-division scheme of the groups. If not, e.g.assuming that there is phase difference equal to period T3, the dutycycle modulation scheme intended for group 1 would e.g. be applied togroup 3.

Frequency Synchronisation

In accordance with the present invention, a frequency synchronisationmay be realised in the following manners:A first method to realise a frequency synchronisation is to synchronisea modulation clock signal of a first control unit with a modulationclock signal of the second control unit.Within the meaning of the present invention, a modulation clock signalof a control unit refers to a repetitive signal that is used for thesynchronisation of actions that are controlled by the control unit, e.g.switching operations or current level adjustments, whereby themodulation clock signal is derived from an internal clock signal of thecontrol unit.Method 1 may thus be described as a method that enables to synchronisethe clock mechanism of both control units on which the modulationrelies. The clock mechanism may be considered the mechanism thatdelivers the modulation clock signal, e.g. an interrupt mechanismdelivering a repetitive interrupt signal based on the internal docksignal of the control unit.As second manner of frequency synchronisation would be to synchronizethe start times of the modulation time windows or sub-windows. As themodulation time windows are arranged head to tail, the stop times arethus known as well. Should there be an intermediate time between cyclesor periods, then the stop times may need a separate synchronisationwhich can be implemented using the same method as used for the starttimes. Note that in general, the modulation cycles or modulation timewindows, periods or sub-windows as applied in the present invention mayhave variable length.More details on both methods are provided here below:

Method 1:

In an embodiment, the first control unit and the second control unit maybe connected by means of a serial communication bus. In an embodiment,each control unit may have a bidirectional or unidirectionalcommunication port for serial communication. As an example, abidirectional communication port of the first control unit may beconnected to a bidirectional communication port of the second controlunit in such a way that bidirectional communication is possible (e.g.Tx1-Rx2 and Rx1-Tx2). In general, a serial communication ischaracterized in that data elements are transmitted consisting of anumber of bits. Each bit represents a state on the communication linewhich is associated with the bit being a logical 1 or a logical 0. Thestate on the communication line can be a voltage, a differentialvoltage, a current, a differential current, an impedance, an opticalintensity, a color, and so on.The serial communication may further be characterised in that each bitof the data elements are present on the communication line for a fixedduration referred to as the bit-time or bit-duration.The first control unit may e.g. be a microprocessor or the like, asknown in the art and operating based on a clock signal, as also known inthe art. The clock signal may be generated external or internal to thefirst control unit and can be characterized by its nominal frequency,actual frequency, jitter, temperature dependency, and so on. As such, atiming based on such a clock signal will vary from processor specimen toprocessor specimen, e.g. depending on the temperature.In order to synchronise the first and second control unit according tothe first method, the first control unit may generate, in a first step,a bit stream for the serial communication based on its clock signalwhereby a duration or period of one bit, i.e. the bit-time orbit-duration, is a large multiple of one period of the clock signal. Inpractice this multiple may be in the range of 200 for an example 8 MHzprocessor and 19200 bit rate. In accordance with this method, the actualbit-time or bit-duration will vary with the actual clock period and thebit-time may exhibit variations over time due to variations over time ofthe clock period and due to effects in the electronics used in thegeneration, such as jitter caused by for example noise on the clocksignal or signal or edge steepness deterioration due to impedancenetworks as posed by communication channels or channel segments.Upon transmission of the bit stream, the second control unit willreceive the bit stream of the serial communication, e.g. via itsbidirectional communication port.In an embodiment of the first frequency synchronisation method, thesecond control unit is configured to measure, in a second step of themethod, the duration or period of 8 data bits by measuring the time fromthe first rising edge of a sent byte 0x55 to the fifth rising edge ofthe start bit following byte 0x55. The resulting duration may then beused, in a third step of the method, to set a counter or timer such thatthe instances at which the incoming data bits are sampled are in themiddle of the data bits received after a synchronization to the signal,e.g. using mechanisms such as pre-ambles, start- and stop bits and thelike and using a first half bit duration offset. This synchronisationmechanism as applied resembles a method typically referred to asautobaud. However, in known, practical implementations of autobaud whichare available on the market, the result of the autobaud measurement ofthe bit-time or bit-duration is not stored with a sufficient resolutionto support a sufficiently accurate synchronisation.In order to obtain a sufficiently high accuracy, in an embodiment of thefirst frequency synchronisation method, referred to as the3-measurement-method, the second control unit measures the length of adata element as transferred over the serial communication line. Apossible implementation is to start the transmission with a start bit,then send the N bits or of the data element and to end with 1 or 2 stopbits. Typically the transferred data element is a byte (N=8). Notehowever, that the data element may also consist of a number of symbols(each consisting of one or more bits), whereby the transmission startswith a start symbol and ends with an end symbol. By measuring the timebetween the rising edge of the start bit of the first byte and the startbit of the second byte, the bit-time or bit-duration can be calculatedusing the total number of start, data and stop bits, and the counter ortimer can be loaded with the correct sample value as explained above. Acondition is that the transmission of the second byte immediatelyfollows that of the first byte.In an embodiment the serial communication may use Manchester encoding.In this embodiment, the second control unit measures the length of fourrising edges in each regular data byte. Using Manchester encoding, eachdata element will either have 3 or four rising edges. The data elementshaving 4 rising edges may have their fourth rising edge at 3 distinctinstances P0, P1 and P2 within the data element duration as isillustrated in FIG. 7.

As the clocks of the first and second control unit are synchronised, themeasurements will form a distribution around the P0, P1 and P2 instanceswith a neglible overlap. This allows the resulting measurement to beclassified as a P0, P1 or P2-measurement and by also taking the positionof the first rising edge into account (derivable from the received dataelement value) the measurement can be converted to a value of abit-duration or a data element-duration.

As can be seen in FIG. 7, data elements having 3 rising edges will nothave a fourth rising edge measurement within the P0, P1 and P2distributions and may be classified as invalid for time measurementafter the P2 distribution's largest time has passed. No timing resultwill be produced for such a data element.As data elements with 4 rising edges will occur regularly in a datastream, measurement results are obtained quite often resulting in aplurality of instants to control and/or adjust the clock mechanism tobecome faster or slower, in order to keep in synchronisation with thefirst control unit. Note that it may not be required to actually adjustthe clock mechanism of the second control unit; based on the determinedbit-time or bit-duration. Based on the measured bit-time, one maydetermine a ratio of the clock periods of both control units and usethis ratio to scale the timing of actions of the second control unit.Note that, as will be understood, that the based on the measurementresults, one may equally adjust the modulation control signal of thefirst control unit to the clock signal of the second control unit.In accordance with an embodiment of the frequency modulation method, themeasurement results as discussed above may be used to adjust thefrequency of the signal delivered by the dock mechanism:In an embodiment the clock mechanism delivers the instruction clocksignal of the processor.Slowing down that clock signal or speeding up that clock signal slowsdown respectively speeds up all operations of the processor and thusalso the modulation clock signal.Slowing down or speeding up the instruction clock signal depends on theprocessor and the clock generation circuitry chosen in the design of thecontrol units. In case of a simple external crystal or ceramicresonator, some tuning can be done by changing the parallel capacitor(s)that is(are) typically connected to the crystal/resonator. Dedicatedelectronics possibly controlled by software can be used to deliver atuneable clock. Another method is using a processor internal tuningmethod.In an embodiment, the clock mechanism delivers a repetitive interruptsignal which determines the timing of the modulation.Typically a repetitive interrupt is generated by configuring theprocessor used to implement the control units to have a timer (workslike a counter) that counts down (the dual case: counting up can beequally well used) from a pre-loaded value and triggers an interruptwhen it rolls over from the 0 value to the pre-loaded value. Theconfiguration during the initialization of the processor than comprisesthe steps of configuring the counter to re-load from a register at thenext active clock edge when at counter value 0, enabling the counter tointerrupt the processor, setting the start value for the counter in theregister and also pre-loading this value in the counter, and to startthe counter.To slow down the interrupt rate during normal operation, the value inthe register can be set to a higher level, to speed up the interruptrate, the value in the register can be set to a lower level, thusslowing down or speeding up the modulation clock signal.In an embodiment, the measured bit-time or bit-duration is multipliedwith a factor and directly loaded in the interrupt timer, via theregister.

Method 2:

In an embodiment of the frequency synchronisation method as applied inthe present invention, the start of the modulation time window or of aperiod or sub-window is made detectable by having the LED drivergenerate a distinct current pulse in the current waveform, the distinctcurrent pulse coinciding in time with the start of the modulation timewindow, period or sub-windowThe detection by the interface board (or LED engine) of the distinctcurrent pulse can either be done directly from a current measurement orfrom the forward voltage resulting from the current flowing through anLED-group.In an embodiment of the present invention, the modulation clock signalor the interrupt signal may be used to synchronise switching operations,e.g. of the LED driver or may be used to initiate the generation of adistinct current pulse.As an example, assume a modulation time window MTW of 3.3. msec, that issubdivided in to 8 sub-windows of 416 μsec.Note that, within the meaning of the present invention, the sub-windowsmay also be referred to as slots or time slots since, in an embodiment,each sub-window or time slot may have a dedicated purpose, i.e. it maybe used to apply a desired current modulation to a particular LED or LEDgroup of the light engine.Further, in the example given, a modulation clock signal or interruptsignal may be generated every 26 μsec, enabling to initiate controlactions every 26 μsec when desired. Note that alternative selections ofthe duration of the modulation time window, the sub-windows or the clocksignal timing may be considered as well.In such example, a distinct current pulse, either an increase in currentor a drop in current may be generated at the start of each modulationtime window MTW, e.g. every 3.3 msec.Such a current pulse may e.g. be detected by means of a currentmeasurement, performed by the light engine or by a voltage detectionacross the LED or LED group that is being powered; more specifically,the application of a current pulse to an LED or LED group will result inan associated pulse of the forward voltage Vf across the LED or LEDgroup.With respect to the application of such current pulses, such a currentpulse may have, in the numeric example as given above, a duration of 26μsec.With respect to the application of current pulses to indicate the startof a modulation time window, it may be pointed out that additionalsynchronisation instants may be obtained, e.g. in case the LED driverdoes not operate in a continuous current mode, whereby the LED driverprovided in a substantially constant current during the entiremodulation time window MTW, as e.g. shown in FIG. 5, but whereby the LEDdriver operates in a duty cycle mode, as e.g. shown in FIG. 6. In casethe LED driver operates in duty cycle mode, the start of each sub-windowmay be apparent from the rising edge of the supply current, i.e. atinstants t=0, t=t1, t=t2 in FIG. 6.

Phase Synchronisation

Within the meaning of the present invention ‘phase synchronisation’ isthe term used to synchronize the second control unit to the firstcontrol unit in such manner that they both work together to achieve thesame overall purpose in each separate modulation cycle, period orsub-period etc.This can be illustrated by the following scheme,

Whereby:

FCU=first control unit,SCU=second control unit,LG1=LED group 1, LG2=LED group 2, and so on.Assuming we have a modulation time window that is subdivided into 4sub-windows or time slots, whereby, during each time slot, the requiredcurrent needs to be generated.The following operating scheme or sequence can be considered anerroneous synchronisation or out-of-phase synchronisation:

FCU: |LG1|LG2|LG3|LG4|LG1| etc SCU: |LG2|LG3|LG4|LG1|LG2| etc

As can be seen from the scheme, in a first time slot, the FCU performscontrol actions for the LG1, while the SCU performs control actions forthe LG2.In case the FCU controls the LED driver and the SCU controls theswitching assembly of the light engine, this situation would correspondto the FCU enabling the LED driver to generate, in the first time slot,the desired current for LED group 1 (LG1), while the SCU controls theswitching assembly such that the supply current (intended for LG1) isprovided to LG2.As will be understood, this would not result in an illuminationcorresponding to a desired illumination as indicated by a received setpoint.The appropriate operating scheme or sequence would have to be:

FCU: |LG1|LG2|LG3|LG4|LG1| etc SCU: |LG1|LG2|LG3|LG4|LG1| etc

In order to ensure that the control units are phase synchronised, in anembodiment, a message may be sent via the serial communication line fromthe first control unit to the second control unit to identify thepurpose or period the first control unit is working on, for example LG2.The second control unit then adapts its actions to also work on overallpurpose P2. After a larger time (e.g. 100 milliseconds dependent onimplementation details, mainly depending on how long the controllersstay in sync) this sync message may be repeated.It may further be noted that, when both control units are in frequencysynchronisation, they can work for a comparatively long time in phasesynchronisation without actively re-syncing them. For the time-framesgiven above, whereby the modulation time window is e.g. 3.3 or 6.6.msec, it may be sufficient to re-synchronise (i.e. re-sync) every 100msec.

In an embodiment, the present invention provides in a light enginecomprising an LED assembly configured to receive a supply current froman LED driver, the LED assembly comprising a plurality of LEDs and oneor more switches arranged in series or in parallel with one or more LEDsof the plurality of LEDs. The light engine further comprises a controlunit configured to control the one or more switches of the LED assembly,thereby controlling an LED current through the plurality of LEDs; thecontrol unit is further configured to:

-   -   receive, at an input terminal, a set point representing a        desired illumination characteristic of an LED assembly of the        light engine    -   determine, based on the received set point, a current amplitude        modulation scheme and a duty cycle modulation scheme;    -   output a first output signal representative of the current        amplitude modulation scheme for processing by an LED driver        control unit of the LED driver;    -   wherein the current amplitude modulation scheme represents an        amplitude of the supply current to be provided by the LED driver        as a function of time within a modulation time window and the        duty cycle modulation scheme represents switching operations for        the one or more switches as a function of time within the        modulation time window and wherein the current amplitude        modulation scheme and the duty cycle modulation scheme are        configured to, when applied by the LED driver control unit and        the control unit, to generate the desired illumination        characteristic.

In accordance with an embodiment of the present invention, the currentamplitude modulation scheme may e.g. be in the form of an array of setpoints (i.e. current set points) to be delivered by an SMPC of an LEDdriver to which the light engine is connected. As such, in anembodiment, the control unit of the light engine assumes the role ofmaster control unit, controlling the switches of the LED assembly and inaddition, outputs a control signal for controlling the LED driver, inparticular an SMPC of the LED driver to which the light engine isconnected or connectable.

The LED driver as applied in the present invention may have multipleoutputs or output channels, each being capable of outputting a supplycurrent with a particular desired amplitude. Such multiple outputs mayduring use e.g. be connected to a respective multiple channels or inputterminals of a light engine, in order to supply multiple groups of LEDsof an LED assembly of a lighting engine.

As required, detailed embodiments of the present invention are disclosedherein: however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention, which can be embodied in variousforms. Therefore, specific structural and functional details disclosedherein are not to be interpreted as limiting, but merely as a basis forthe claims and as a representative basis for teaching one skilled in theart to variously employ the present invention in virtually anyappropriately detailed structure. Further, the terms and phrases usedherein are not intended to be limiting, but rather, to provide anunderstandable description of the invention.

The terms “a” or “an”, as used herein, are defined as one or more thanone. The term plurality, as used herein, is defined as two or more thantwo. The term another, as used herein, is defined as at least a secondor more. The terms including and/or having, as used herein, are definedas comprising (i.e., open language, not excluding other elements orsteps). Any reference signs in the claims should not be construed aslimiting the scope of the claims or the invention.

The mere fact that certain measures are recited in mutually differentdependent claims does not indicate that a combination of these measurescannot be used to advantage.

The term coupled, as used herein, is defined as connected, although notnecessarily directly, and not necessarily mechanically.

A single processor or other unit may fulfil the functions of severalitems recited in the claims.

The terms program, software application, and the like as used herein,are defined as a sequence of instructions designed for execution on acomputer system. A program, computer program, or software applicationmay include a subroutine, a function, a procedure, an object method, anobject implementation, an executable application, an applet, a servlet,a source code, an object code, a shared library/dynamic load libraryand/or other sequence of instructions designed for execution on acomputer system.

A computer program may be stored and/or distributed on a suitablemedium, such as an optical storage medium or a solid-state mediumsupplied together with or as part of other hardware, but also bedistributed in other forms, such as via the Internet or other wired orwireless telecommunication systems.

1-36. (canceled)
 37. A modular system comprising a first componentcomprising an LED driver and a second component comprising a lightengine, the LED driver comprising: a switched mode power converterconfigured to output a supply current; a first control unit configuredto control a switch of the switched mode power converter, therebycontrolling the supply current; the light engine comprising: an LEDassembly configured to receive the supply current, the LED assemblycomprising a plurality of LEDs or LED groups and one or more switchesarranged in series or in parallel with one or more LEDs or LED groups ofthe plurality of LEDs of LED groups, and a second control unitconfigured to control the one or more switches of the LED assembly,thereby controlling a supply of the supply current to the plurality ofLEDs or LED groups of the LED assembly; wherein the first control unitand the second control unit are configured to co-operate and control thesupply current and the supply of the supply current to the LED assemblyin accordance with a desired illumination characteristic, and whereinthe first and second control unit are configured to synchronize aswitching operation of the switch of the switched mode power converterwith a switching operation of the one or more switches of the lightengine, wherein the first control unit and the second control unit areconfigured to control the LED assembly to generate a desiredillumination characteristic by controlling the switch and the one ormore switches within a predetermined modulation time window and whereinthe first control unit is configured to: receive, at an input terminalof the first control unit, an input signal representing the desiredillumination characteristic; determine, based on the desiredillumination characteristic, a current amplitude modulation scheme, aduty cycle modulation scheme and a time-division scheme; provide anoutput signal representative of the time-division scheme to the secondcontrol unit; wherein the first control unit is configured to apply thecurrent amplitude modulation scheme and the duty cycle modulation schemewithin the modulation time window and wherein the second control unit isconfigured to apply the time-division scheme within the modulation timewindow, in order to generate the desired illumination characteristic.38. The modular system according to claim 37, wherein the plurality ofLEDs is arranged in groups of LEDs in series, each of the groups of LEDsbeing provided with a switch of the one or more switches, the switchbeing connected in parallel to the respective group of LEDs.
 39. Themodular system according to claim 37, wherein the plurality of LEDs isarranged in groups of LEDs arranged in parallel, each of the groups ofLEDs being provided with a switch of the one or more switches, theswitch being connected in series with the respective group of LEDs. 40.The modular system according to claim 37, wherein the current amplitudemodulation scheme represents an amplitude of the supply current as afunction of time within the modulation time window.
 41. The modularsystem according to claim 37, wherein the duty cycle modulation schemerepresents switching operations for the switch of the switched modepower supply as a function of time within the modulation time window.42. The modular system according to claim 37, wherein the time-divisionscheme represents a subdivision of the modulation time window into aplurality of sub-windows, whereby each of the plurality of groups ofLEDs is provided with the supply current during only one sub-window ofthe plurality of sub-windows.
 43. The modular system according to claim42, wherein the subdivision of the modulation time window into theplurality of sub-windows is based on a desired color of the illuminationcharacteristic.
 44. The modular system according to claim 42, whereinthe sub-windows have the same duration.
 45. A modular system comprisinga first component comprising an LED driver and a second componentcomprising a light engine, the LED driver comprising: a switched modepower converter configured to output a supply current; a first controlunit configured to control a switch of the switched mode powerconverter, thereby controlling the supply current; the light enginecomprising: an LED assembly configured to receive the supply current,the LED assembly comprising a plurality of LEDs or LED groups and one ormore switches arranged in series or in parallel with one or more LEDs orLED groups of the plurality of LEDs of LED groups, and a second controlunit configured to control the one or more switches of the LED assembly,thereby controlling a supply of the supply current to the plurality ofLEDs or LED groups of the LED assembly; wherein the first control unitand the second control unit are configured to co-operate and control thesupply current and the supply of the supply current to the LED assemblyin accordance with a desired illumination characteristic, and whereinthe first and second control unit are configured to synchronize aswitching operation of the switch of the switched mode power converterwith a switching operation of the one or more switches of the lightengine, wherein the first control unit and the second control unit areconfigured to control the LED assembly to generate a desiredillumination characteristic by controlling the switch and the one ormore switches within a predetermined modulation time window and whereinthe second control unit is configured to: receive, at an input terminalof the second control unit, an input signal representing the desiredillumination characteristic; determine, based on the desiredillumination characteristic, a current amplitude modulation scheme and aduty cycle modulation scheme; provide an output signal representative ofthe current amplitude modulation scheme to the first control unit;wherein the first and second control unit are respectively configured toapply the current amplitude modulation scheme and the duty cyclemodulation scheme within the modulation time window, in order togenerate the desired illumination characteristic.
 46. The modular systemaccording to claim 45, wherein the current amplitude modulation schemerepresents an amplitude of the supply current as a function of timewithin the modulation time window.
 47. The modular system according toclaim 45, wherein the duty cycle modulation scheme represents switchingoperations for the one or more switches as a function of time within themodulation time window.
 48. The modular system according to claim 47,wherein amplitude modulations of the current amplitude modulation schemeand switching operations of the duty cycle modulation scheme arenon-overlapping.
 49. The modular system according to claim 45, whereinthe plurality of LEDs are arranged in two or more groups, each grouphaving a switch associated with it to control a current through therespective group, the duty cycle modulation scheme comprising a groupduty cycle modulation scheme for each of the two or more groups andwherein switching operations of the two or more group duty cyclemodulation schemes are non-overlapping.
 50. The modular system accordingto claim 45, wherein the modulation time window is equal to 3.3 ms or amultiple thereof.
 51. The modular system according to claim 45, whereinthe first and second control unit are configured to synchronize aswitching operation of the switch of the switched mode power converterwith a switching operation of the one or more switches of the lightengine by receiving a common clock signal.
 52. The modular systemaccording to claim 45, wherein the first and second control unit areconfigured to synchronize a switching operation of the switch of theswitched mode power converter with a switching operation of the one ormore switches of the light engine by exchanging a synchronizationsignal.
 53. The modular system according to claim 52, wherein the secondcontrol unit is configured to provide the synchronization signal to thefirst control unit, the second control unit being configured to resetthe switch mode power converter upon receipt of the synchronizationsignal.
 54. The modular system according to claim 45, further comprisinga serial communication link for connecting the first control unit to thesecond control unit.
 55. The modular system according to claim 54,wherein the first control unit is configured to control the switch ofthe switched mode power converter using a modulation clock signal of thefirst control unit, whereby the modulation clock signal is based on aclock signal of the first control unit.
 56. The modular system accordingto claim 55, wherein the first control unit is configured to transmit,via the serial communication link, a data element comprising a pluralityof bits to the second control unit and wherein the second unit isconfigured to, upon receipt of the data element, determine a duration ofthe transmitting of the data element and synchronise a clock signal ofthe second control unit based on the duration.
 57. The modular systemaccording to claim 56, wherein the data element is Manchester encoded.58. The modular system according to claim 45, wherein the first controlunit is configured to control the switched mode power supply to generatea current pulse at a predetermined period, and wherein the light engineis configured to detect a timing of the current pulse and synchronize acontrol of the one or more switches based on the timing of the currentpulse.
 59. The modular system according to claim 58, wherein the lightengine comprises a measurement unit configured to perform a currentmeasurement of the supply current or a forward voltage measurement andprovide a measurement signal representing the current or forward voltagemeasurement to the second control unit to determine the timing of thecurrent pulse.
 60. The modular system according to claim 45, wherein thefirst and second control unit are configured to perform a frequencysynchronisation and a phase synchronisation of the current amplitudemodulation scheme, the duty cycle modulation scheme and thetime-division scheme.
 61. The modular system according to claim 60,further comprising a serial communication link for connecting the firstcontrol unit to the second control unit, and wherein the second controlunit is configured to perform the phase synchronisation based on asignal received from the first control unit, via the serialcommunication link.
 62. The modular system according to 45, wherein thesecond control unit is configured to determine a timing of the switchingoperation of the switch of the switched mode power converter based on acurrent measurement of the supply current.
 63. The modular systemaccording to claim 62, wherein the light engine comprises a measurementunit configured to perform the current measurement of the supply currentand provide a current measurement signal representing the currentmeasurement to the second control unit to determine the timing of theswitching operation of the switch.
 64. A modular system comprising afirst component comprising an LED driver and a second componentcomprising a light engine, the LED driver comprising: a switched modepower converter configured to output a supply current; a first controlunit configured to control a switch of the switched mode powerconverter, thereby controlling the supply current; the light enginecomprising: an LED assembly configured to receive the supply current,the LED assembly comprising a plurality of LEDs and one or more switchesarranged in series or in parallel with one or more LEDs of the pluralityof LEDs, and a second control unit configured to control the one or moreswitches of the LED assembly, thereby controlling an LED current throughthe plurality of LEDs; the system further comprising a main control unitconfigured to: receive, at an input terminal, a set point representing adesired illumination characteristic of the LED assembly; determine,based on the received set point, a current amplitude modulation schemeand a duty cycle modulation scheme; provide a first output signalrepresentative of the current amplitude modulation scheme to the firstcontrol unit and a second output signal representative of the duty cyclemodulation scheme to the second control unit; wherein the first andsecond control unit are respectively configured to apply the currentamplitude modulation scheme and the duty cycle modulation scheme withina modulation time window, in order to generate the desired illuminationcharacteristic.
 65. A light engine comprising: an LED assemblyconfigured to receive a supply current from an LED driver, the LEDassembly comprising a plurality of LEDs and one or more switchesarranged in series or in parallel with one or more LEDs of the pluralityof LEDs, and a control unit configured to control the one or moreswitches of the LED assembly, thereby controlling an LED current throughthe plurality of LEDs; the control unit further being configured to:receive, at an input terminal, a set point representing a desiredillumination characteristic of the LED assembly; determine, based on thereceived set point, a current amplitude modulation scheme and a dutycycle modulation scheme; output a first output signal representative ofthe current amplitude modulation scheme for processing by an LED drivercontrol unit of the LED driver; wherein the current amplitude modulationscheme represents an amplitude of the supply current to be provided bythe LED driver as a function of time within a modulation time window andthe duty cycle modulation scheme represents switching operations for theone or more switches as a function of time within the modulation timewindow and wherein the current amplitude modulation scheme and the dutycycle modulation scheme are configured to, when applied by the LEDdriver control unit and the control unit, to generate the desiredillumination characteristic.
 66. The light engine according to claim 65,wherein amplitude modulations of the current amplitude modulation schemeand switching operations of the duty cycle modulation scheme arenon-overlapping.
 67. The light engine according to any of the claim 65,wherein the light engine comprises a measurement unit configured toperform, during use, a current measurement of the supply current andprovide a current measurement signal representing the currentmeasurement to the second control unit to determine a timing of aswitching operation of a switch of the LED driver, the light enginefurther being configured to synchronize a switching operation of the oneor more switches of the light engine with the timing of the switchingoperation of the switch.
 68. An LED driver comprising: a switched modepower converter configured to output a supply current for powering anLED assembly; a control unit configured to control a switch of theswitched mode power converter, thereby controlling the supply current;wherein the control unit is further configured to: receive, at an inputterminal of the control unit, LED assembly information describing theLED assembly to be powered; receive, at the input terminal, a set pointrepresenting a desired illumination characteristic to be generated,during use, by the LED assembly; determine, based on the received setpoint, a current amplitude modulation scheme and a duty cycle modulationscheme; output a first output signal representative of the duty cyclemodulation scheme for processing by an LED assembly control unit of theLED assembly that is to be powered; wherein the current amplitudemodulation scheme represents an amplitude of the supply current to beprovided by the LED driver as a function of time within a modulationtime window and the duty cycle modulation scheme represents switchingoperations for the LED assembly as a function of time within themodulation time window and wherein the current amplitude modulationscheme and the duty cycle modulation scheme are configured to, whenapplied by the LED assembly control unit and the control unit, togenerate the desired illumination characteristic.
 69. The LED driveraccording to claim 68, wherein amplitude modulations of the currentamplitude modulation scheme and switching operations of the duty cyclemodulation scheme are non-overlapping.